Manage your subscription

Puffed-up planet puzzles astronomers

By Hazel Muir

An amazingly swollen planet has been spotted circling a star in the constellation Lacerta. It is the second of its kind, which makes astronomers suspect these inexplicably puffed-up worlds are actually common.

The bloated planet HAT-P-1 has a radius about 1.38 times that of Jupiter but contains only half Jupiter’s mass (Illustration: David A. Aguilar/CfA)

The average density of the new planet is about a quarter that of water, making it less dense than a wine cork. The team who discovered it, led by Gáspár Bakos from the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts, US, reported the discovery on Thursday at a press conference in Washington DC, US.

“If Saturn was to float in water, it would be like an iceberg with most of the planet under the water,” team member Dimitar Sasselov from the CfA told New Scientist. “But our new planet would stick out on top like a beachball.”

The team first spotted the planet using the Hungarian Automated Telescope (HAT). HAT is a network of six small telescopes in Arizona and Hawaii that use off-the-shelf camera lenses to monitor wide areas of sky to look for tiny brightness changes in stars.

Advertisement

Computer analysis of long-term observations by HAT revealed that one of a pair of Sun-like stars 450 light years away dims by about 1.5% every 4.5 days. Follow-up observations this summer using large telescopes in Hawaii confirmed that the dimming is due to a planet half as massive as Jupiter passing or “transiting” across the star’s face during its orbit.

Obesity epidemic

But the amount of dimming only makes sense if the planet, now called HAT-P-1, is actually 38% wider than Jupiter, making it very bloated. That is partly because it is baking in the heat close to its star. Even so, it is still 24% bigger than theory predicts.

A similar planet, HD 209458b, was discovered in 1999. It is 20% larger than theory predicts. Since only 11 transiting planets are known in total, the fact that two are bloated suggests this is common.

“Whatever the process that makes planets like this, it is not particularly rare,” comments Tim Brown, an extrasolar planet expert at the National Center for Atmospheric Research in Boulder, Colorado, US. “But I don’t think anybody understands why HD 209458b or HAT-P-1 are so big.”

The planets might have some kind of internal heat source that puffs them up. One possibility is tidal heating. The gravity of each planet’s parent star might distort and stretch it, dissipating energy as internal heat.

Curious and unlikely

However, astronomers cannot make this theory work without giving both planets curious and unlikely spin-axis orientations or highly elongated orbits. They know that the orbit of HD 209458b, at least, is not very elongated.

Sasselov says there’s another possibility – that some poorly understood mechanism has separated hydrogen and helium in each planet. “Being heavier, helium would then settle to the centre of the planet,” says Sasselov. “That contraction could release gravitational energy as good old-fashioned heat.” If so, then astronomers would have to explain why that does not happen to all giant planets.

Brown concludes that HAT-P-1 will give astronomers plenty to chew on. “The result is a fascinating and difficult challenge to theorists, who need to explain how planets like these can form, and also to observers, whose job it is to quash such theories when they are clever but wrong,” he says.